1. Field of the Invention
This invention relates generally to radio receivers and, more particularly to high definition (HD) radio receivers.
2. Background of the Invention
Referring to FIG. 1, the structure of a high definition (HD) radio signal is illustrated. The center frequency band 10 is typically an analog signal and the sideband signals 11A and 11B are typically digital signals. In the following discussion, narrowband and broadband regions are indicated. A narrowband region has the frequency range to include only the center frequency band 10, while the broadband region is sufficiently wide to include the center frequency band 10 and the sidebands 11A and 11B. In the HD radio system, the program information is transmitted in the center band frequency spectrum, while related information, including possibly a simulcast, is transmitted by the sidebands.
After receiving a broadcast signal, the frequency of the received signal is down-converted by mixing the broadcast signal with a local oscillator to generate an intermediate frequency (IF) signal. An HD radio receiver typically employs a 180 kHz “narrowband” IF filter when the analog FM signal is being received. The narrowband filter is used to minimize desensitization of the FM signal resulting from out-of-band interference signals. The HD radio receiver must use a 550 kHz “wideband” IF filter when an HD radio station is received in order that the digital sidebands of the HD radio signal will fall within the bandwidth of the IF filter. The sidebands of the HD radio signal would otherwise fall outside of the bandwidth of the narrowband filter, which is optimum only for the bandwidth of analog FM signal. Typically, switching is provided between the wideband and the narrowband IF filters when attempting to detect the acquisition or loss of the HD radio signal. Furthermore, when an analog FM signal is being received, the receiver should “hunt” for the digital sidebands of the HD radio signal bands by periodically switching from the narrowband IF filter to the wideband IF filter. When no HD radio sidebands are detected, then the receiver should switch back to the narrowband IF filter.
Unfortunately, the switching between narrow band and wideband IF filters is not without problems. For instance, when the receiver switches from the narrowband IF filter to the wideband IF filter to hunt for the HD sidebands, the analog signal can be desensitized by adjacent noise falling within the bandwidth of the wideband filter. In more detail, a strong interference signal falling within the bandwidth of the wideband filter can cause the AGC (automatic gain control) function within the tuner to reduce the gain of the IF signal. The result is a reduction of the SNR (signal-to-noise ratio) of the desired signal in the presence of a strong interfering signal. To the person listening to the radio, the effect is a periodic disturbance in the audio, similar to a burst of noise.
Some systems employ two IF signal paths to avoid this problem. One path is filtered by the wideband filter and the other path is filtered by a narrowband IF filter. Since the two paths are independent, no filter switching is required and the audio is not disturbed when the receiver hunts for the digital sidebands of the HD radio signal. A system with two IF signal paths is more expensive than a system with only one IF signal path because a high quality ADC is required to digitize both IF signals. Two high quality ADCs require additional expense.
However, many automotive radio receiver manufacturers use a dual-tuner apparatus to improve the quality of the analog radio signals. This approach is independent of the HD radio and does not apply to the reception of the HD radio signals. The dual tuner (also called tuner diversity) technique mixes the signals from two tuners, each fed by a separate antenna input, usually from spatially distant areas on the vehicle, to form a combined signal with higher quality than either of the two individual signals alone. Since a dual-tuner receiver requires two IF paths (and two tuners), it is more expensive than a single-tuner system.
Referring to FIG. 2, the implementation of the HD radio receiver 20 according to the prior art is shown. In a first path, an antenna 21 receives the HD radio signal. This signal is applied to a tuner 22, including broadband filter 221, narrowband filter 222 and switch 223. The signal from tuner 22 is applied to digitizer 24. The output signal from digitizer 24 is applied to the digital signal processor (DSP) 25 wherein the signal is processed and applied to loudspeaker 26. A second HD radio signal path includes antenna 21′, tuner 22′, tuner 21′ including narrowband filter 222; and digitizer 24′ before being applied to digital signal processor 25. Switch 223 includes apparatus for determining when the signal in the broadband filter signal has been compromised and switches the signal applied to the digital signal processor to the signal from the narrowband filter. When the signal in the broadband filter path is no longer compromised, the signal applied to the digital signal processor by the digitizer 24 is signal from the broadband filter path. In the processor 25, the higher quality center frequency hand signal from digitizer 24 and digitizer 24′ is applied to speaker 25′.
Referring to FIG. 3, a second implementation of the prior art HD radio receiver is shown. In this implementation, the output signals from the broadband filter tuner 321 or the narrowband filter tuner 322 are selected by switch 323 and applied to digitizer 34. The digitized signal is applied to the digital signal processor 35 and then to the speaker 36. When the signal through the broadband filter 321 is comprised, only the signal from the narrowband filter tuner 322′ is applied to the digital signal processor 36.
The HD radio reception capability is being added to high-quality radios that use the dual tuner technique for improved reception of the analog FM signal. If only two IF paths are used in such a system, the system will suffer from periodic disturbances in the audio output signals resulting from the switching the wideband and narrow band filters in one of the tuner paths while it hunts for the digital sidebands of the HD radio signal. The problem could be solved by adding a third IF path, but at an increased cost.
Summarizing, the HD radio receiver will typically process the signal passing through the broadband filter tuner. In this manner, the center frequency bands and the sidebands are processed. When the signal through the broadband tuner is degraded, the signal from the narrowband filter tuner is processed. This transition can result in an undesirable audio output resulting from rapid transients.
A need has therefore been felt for apparatus and an associated method having the feature of improving the audio output signal of a HD radio receiver. It would be yet another feature of the apparatus and associated method to improve the switching between a narrowband filter signal path and broadband filter signal path on the audio output signal.